An acceleration sensor of this kind is described in U.S. Pat. No. 4,827,091. This known sensor comprises a cylinder made of a conductive material, a magnetized inertial member mounted in the cylinder so as to be movable longitudinally of the cylinder, a conductive member mounted at least on an end surface of the inertial member which is on a side of one longitudinal end of the cylinder, a pair of electrodes disposed at the one longitudinal end of the cylinder, and an attracting member disposed near the other longitudinal end of the cylinder When the conductive member of the magnetized inertial member makes contact with the electrodes, these electrodes are caused to conduct via the conductive member. The attracting member is made of such a magnetic material that the attracting member and the inertial member are magnetically attracted towards each other.
In this acceleration sensor, the magnetized inertial member and the attracting member attract each other. When no or almost no acceleration is applied to the sensor, the inertial member is at rest at the other end in the cylinder.
If a relatively large acceleration acts on this acceleration sensor, the magnetized inertial member moves against the attracting force of the attracting member. During the movement of the inertial member, an electrical current is induced in this cylinder, producing a magnetic force which biases the inertial member in the direction opposite to the direction of movement of the inertial member. Therefore, the magnetized inertial member is braked, so that the speed of the movement is reduced.
When the acceleration is less than a predetermined magnitude, or threshold value, the magnetized inertial member comes to a stop before it reaches the front end of the cylinder. Then, the inertial member is pulled back by the attracting force of the attracting member.
When the acceleration is greater than the predetermined magnitude, or the threshold value, e.g., the vehicle carrying this acceleration sensor collides with an object, the inertial member arrives at the one end or front end of the cylinder. At this time, the conductive layer on the front end surface of the inertial member makes contact with both electrodes to electrically connect them with each other. If a voltage has been previously applied between the electrodes, an electrical current flows when a short circuit occurs between them. This electrical current permits detection of collision of the vehicle.
As shown in FIG. 2, the conventional magnetized inertial member 1 consists of a magnet assembly comprising a permanent magnet 2 enclosed in a case 3 made of copper. A packing 4 is made of a synthetic resin. This case 3 permits the magnetized inertial member 1 to smoothly slide along the inner surface of the cylinder. If the vehicle collides with an object, the inertial member 1 receives an acceleration. At this time, the inertial member 1 moves and allows the case to contact with the electrodes, thus causing them to conduct, i.e., they are short-circuited.
When the conventional magnetized inertial member 1 shown in FIG. 2 is assembled, the magnet 2 is inserted into the case 3. Then, the packing 4 is loaded into it. Subsequently, one end portion of the case 3 is bent inwardly. In this way, laborious steps are needed.